Swift：Apple的下一代主流编程语言

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"Apple的Swift编程语言是专为iOS和OS X应用设计的，它结合了C和Objective-C的优点，并且摆脱了C语言兼容性的束缚。Swift采用安全的编程模式，添加了现代特性，使编程更简单、更灵活、更有趣。作为Cocoa和Cocoa Touch框架的坚实后盾，Swift提供了重新思考软件开发方式的机会。经过多年的研发，Swift在Apple现有编译器、调试器和框架基础设施的基础上构建，如Automatic Reference Counting (ARC)的引入，以及Objective-C对现代语言技术的支持，如blocks、集合字面量和模块。这些准备工作使得推出Swift这种面向未来苹果软件开发的新语言成为可能。对于Objective-C开发者来说，Swift有很强的亲切感，它继承了Objective-C命名参数的可读性。" 详细知识点: 1. **Swift编程语言**：Swift是Apple公司推出的一种新型编程语言，专为iOS、macOS、watchOS和tvOS平台的应用程序设计，旨在提供更加高效、安全和现代化的编程体验。 2. **面向未来**：Swift摒弃了对C语言的兼容性需求，这使得它能够采用更现代的编程范式和特性，为软件开发带来革新。 3. **安全的编程模式**：Swift通过内置的安全机制，如强类型、模式匹配、可选类型等，降低了编程中的错误概率，提高了代码的可靠性。 4. **现代特性**：Swift支持闭包（closures）、泛型（generics）、枚举（enums）和结构体（structs），这些都让编程更加灵活，同时增加了函数式编程元素。 5. **Automatic Reference Counting (ARC)**：Swift采用了自动引用计数，这是一种内存管理机制，自动处理对象的生命周期，避免了内存泄漏和意外的空指针异常。 6. **Cocoa和Cocoa Touch**：Swift与Apple的成熟框架Cocoa和Cocoa Touch紧密集成，为开发者提供了丰富的API和工具，简化了应用程序的开发过程。 7. **Objective-C兼容性**：尽管Swift是一个新的语言，但它在语法上与Objective-C有一定的相似性，特别是命名参数的使用，这使得Objective-C开发者可以更容易地过渡到Swift。 8. **模块系统**：Swift支持模块，允许代码的组织和重用更加高效，也方便了库和框架的构建。 9. **持续演进**：Apple对Swift的持续更新和改进表明，该语言会不断适应开发者的需求和技术的发展，保持其在Apple生态系统中的领先地位。 10. **学习曲线**：对于熟悉Objective-C的开发者来说，Swift的学习曲线相对平缓，因为它们之间有许多共同的概念和语法，这促进了开发者快速掌握新语言。 Swift编程语言是Apple为了提高开发者效率和软件质量而设计的，它的出现为iOS和macOS开发带来了全新的体验，同时鼓励开发者采用更加安全、现代的编程实践。

Experiment

Write an enumeration that conforms to this protocol.

Notice the use of the mutating keyword in the declaration of SimpleStructure to mark a

method that modifies the structure. The declaration of SimpleClass doesn’t need any of its

methods marked as mutating because methods on a class can always modify the class.

Use extension to add functionality to an existing type, such as new methods and computed

properties. You can use an extension to add protocol conformance to a type that is declared

elsewhere, or even to a type that you imported from a library or framework.

 extension Int: ExampleProtocol {

 var simpleDescription: String {

 return "The number \(self)"

 }

 mutating func adjust() {

 self += 42

 }

 7.simpleDescription

Experiment

Write an extension for the Double type that adds an absoluteValue property.

You can use a protocol name just like any other named type—for example, to create a

collection of objects that have different types but that all conform to a single protocol. When

you work with values whose type is a protocol type, methods outside the protocol definition

are not available.

 let protocolValue: ExampleProtocol = a

 protocolValue.simpleDescription

 // protocolValue.anotherProperty // Uncomment to see the error

Even though the variable protocolValue has a runtime type of SimpleClass, the compiler

treats it as the given type of ExampleProtocol. This means that you can’t accidentally access

methods or properties that the class implements in addition to its protocol conformance.

Generics

Write a name inside angle brackets to make a generic function or type.

 func repeat<ItemType>(item: ItemType, times: Int) -> [ItemType] {

 var result = [ItemType]()

 for i in 0..<times {

 result.append(item)

 }

 return result

The Basics

Swift is a new programming language for iOS and OS X app development. Nonetheless,

many parts of Swift will be familiar from your experience of developing in C and Objective-

Swift provides its own versions of all fundamental C and Objective-C types, including Int

for integers, Double and Float for floating-point values, Bool for Boolean values, and

String for textual data. Swift also provides powerful versions of the two primary collection

types, Array and Dictionary, as described in Collection Types.

Like C, Swift uses variables to store and refer to values by an identifying name. Swift also

makes extensive use of variables whose values cannot be changed. These are known as

constants, and are much more powerful than constants in C. Constants are used throughout

Swift to make code safer and clearer in intent when you work with values that do not need to

change.

In addition to familiar types, Swift introduces advanced types not found in Objective-C, such

as tuples. Tuples enable you to create and pass around groupings of values. You can use a

tuple to return multiple values from a function as a single compound value.

Swift also introduces optional types, which handle the absence of a value. Optionals say

either “there is a value, and it equals x” or “there isn’t a value at all”. Optionals are similar to

using nil with pointers in Objective-C, but they work for any type, not just classes. Optionals

are safer and more expressive than nil pointers in Objective-C and are at the heart of many

of Swift’s most powerful features.

Optionals are an example of the fact that Swift is a type safe language. Swift helps you to be

clear about the types of values your code can work with. If part of your code expects a

String, type safety prevents you from passing it an Int by mistake. This restriction enables

you to catch and fix errors as early as possible in the development process.

Constants and Variables

Constants and variables associate a name (such as maximumNumberOfLoginAttempts or

welcomeMessage) with a value of a particular type (such as the number 10 or the string

"Hello"). The value of a constant cannot be changed once it is set, whereas a variable can be

set to a different value in the future.

Declaring Constants and Variables

Constants and variables must be declared before they are used. You declare constants with

the let keyword and variables with the var keyword. Here’s an example of how constants

and variables can be used to track the number of login attempts a user has made:

 let maximumNumberOfLoginAttempts = 10

 var currentLoginAttempt = 0

This code can be read as:

Note

It is rare that you need to write type annotations in practice. If you provide an initial value for

a constant or variable at the point that it is defined, Swift can almost always infer the type to

be used for that constant or variable, as described in Type Safety and Type Inference. In the

welcomeMessage example above, no initial value is provided, and so the type of the

welcomeMessage variable is specified with a type annotation rather than being inferred from

an initial value.

Naming Constants and Variables

Constant and variable names can contain almost any character, including Unicode characters:

 let π = 3.14159

 let 你好 = "你好世界"

 let ?? = "dogcow"

Constant and variable names cannot contain whitespace characters, mathematical symbols,

arrows, private-use (or invalid) Unicode code points, or line- and box-drawing characters.

Nor can they begin with a number, although numbers may be included elsewhere within the

name.

Once you’ve declared a constant or variable of a certain type, you can’t redeclare it again

with the same name, or change it to store values of a different type. Nor can you change a

constant into a variable or a variable into a constant.

Note

If you need to give a constant or variable the same name as a reserved Swift keyword,

surround the keyword with back ticks (`) when using it as a name. However, avoid using

keywords as names unless you have absolutely no choice.

You can change the value of an existing variable to another value of a compatible type. In

this example, the value of friendlyWelcome is changed from "Hello!" to "Bonjour!":

 var friendlyWelcome = "Hello!"

 friendlyWelcome = "Bonjour!"

 // friendlyWelcome is now "Bonjour!"

Unlike a variable, the value of a constant cannot be changed once it is set. Attempting to do

so is reported as an error when your code is compiled:

 let languageName = "Swift"

 languageName = "Swift++"

 // this is a compile-time error - languageName cannot be changed

Printing Constants and Variables

You can print the current value of a constant or variable with the println function:

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